The invention relates to a circuit arrangement for a small private automatic branch exchange (PABX), in which, with the aid of a control computer unit that directs the switching processes and has access to memory units in which the information necessary for the switching processes is contained, internal and external connections are made possible by the interposition of an appropriate switching matrix with electronic crosspoints.
Through the use of a control computer unit, which can take the form of a microprocessor that operates together with the corresponding storage systems, a wide range of features can be obtained inexpensively in a small PABX. Examples of these features include abbreviated dialing, which enables authorized subscribers to establish a frequently used connection by dialing an abbreviated number or code, repeat dialing, by means of which the outside number previously dialed is called again, call-waiting signals which allow a person to draw attention to himself by actuating a tone when a line is busy, as well as call forwarding, in which an incoming call is routed to another predetermined subscriber station. These and other features can be changed by programming, which is accomplished with appropriate input keys, assuming that the system is software controlled. This allows the system to be easily and simply adapted or customized for the requirements of a particular user. In addition to electromechanical elements, semiconductor elements, such as field-effect transistors, are used as crosspoints in the switching matrix. Even when the switching matrix is constructed in a single stage, there are, as a rule, at least two crosspoints in series for any internal connection. If electronic crosspoints are used, it is possible, among other things, to reduce the noise generated in the PABX.